diff options
| -rw-r--r-- | include/qapi/error.h | 158 |
1 files changed, 139 insertions, 19 deletions
diff --git a/include/qapi/error.h b/include/qapi/error.h index 2c189abb04..85df875a3a 100644 --- a/include/qapi/error.h +++ b/include/qapi/error.h @@ -30,6 +30,10 @@ * job. Since the value of @errp is about handling the error, the * function should not examine it. * + * - The function may pass @errp to functions it calls to pass on + * their errors to its caller. If it dereferences @errp to check + * for errors, it must use ERRP_GUARD(). + * * - On success, the function should not touch *errp. On failure, it * should set a new error, e.g. with error_setg(errp, ...), or * propagate an existing one, e.g. with error_propagate(errp, ...). @@ -45,15 +49,17 @@ * = Creating errors = * * Create an error: - * error_setg(&err, "situation normal, all fouled up"); + * error_setg(errp, "situation normal, all fouled up"); + * where @errp points to the location to receive the error. * * Create an error and add additional explanation: - * error_setg(&err, "invalid quark"); - * error_append_hint(&err, "Valid quarks are up, down, strange, " + * error_setg(errp, "invalid quark"); + * error_append_hint(errp, "Valid quarks are up, down, strange, " * "charm, top, bottom.\n"); + * This may require use of ERRP_GUARD(); more on that below. * * Do *not* contract this to - * error_setg(&err, "invalid quark\n" // WRONG! + * error_setg(errp, "invalid quark\n" // WRONG! * "Valid quarks are up, down, strange, charm, top, bottom."); * * = Reporting and destroying errors = @@ -107,18 +113,6 @@ * Errors get passed to the caller through the conventional @errp * parameter. * - * Pass an existing error to the caller: - * error_propagate(errp, err); - * where Error **errp is a parameter, by convention the last one. - * - * Pass an existing error to the caller with the message modified: - * error_propagate_prepend(errp, err, - * "Could not frobnicate '%s': ", name); - * This is more concise than - * error_propagate(errp, err); // don't do this - * error_prepend(errp, "Could not frobnicate '%s': ", name); - * and works even when @errp is &error_fatal. - * * Create a new error and pass it to the caller: * error_setg(errp, "situation normal, all fouled up"); * @@ -129,18 +123,26 @@ * handle the error... * } * - when it does not, say because it is a void function: + * ERRP_GUARD(); + * foo(arg, errp); + * if (*errp) { + * handle the error... + * } + * More on ERRP_GUARD() below. + * + * Code predating ERRP_GUARD() still exists, and looks like this: * Error *err = NULL; * foo(arg, &err); * if (err) { * handle the error... - * error_propagate(errp, err); + * error_propagate(errp, err); // deprecated * } - * Do *not* "optimize" this to + * Avoid in new code. Do *not* "optimize" it to * foo(arg, errp); * if (*errp) { // WRONG! * handle the error... * } - * because errp may be NULL! + * because errp may be NULL without the ERRP_GUARD() guard. * * But when all you do with the error is pass it on, please use * foo(arg, errp); @@ -160,6 +162,19 @@ * handle the error... * } * + * Pass an existing error to the caller: + * error_propagate(errp, err); + * This is rarely needed. When @err is a local variable, use of + * ERRP_GUARD() commonly results in more readable code. + * + * Pass an existing error to the caller with the message modified: + * error_propagate_prepend(errp, err, + * "Could not frobnicate '%s': ", name); + * This is more concise than + * error_propagate(errp, err); // don't do this + * error_prepend(errp, "Could not frobnicate '%s': ", name); + * and works even when @errp is &error_fatal. + * * Receive and accumulate multiple errors (first one wins): * Error *err = NULL, *local_err = NULL; * foo(arg, &err); @@ -187,6 +202,69 @@ * error_setg(&err, ...); // WRONG! * } * because this may pass a non-null err to error_setg(). + * + * = Why, when and how to use ERRP_GUARD() = + * + * Without ERRP_GUARD(), use of the @errp parameter is restricted: + * - It must not be dereferenced, because it may be null. + * - It should not be passed to error_prepend() or + * error_append_hint(), because that doesn't work with &error_fatal. + * ERRP_GUARD() lifts these restrictions. + * + * To use ERRP_GUARD(), add it right at the beginning of the function. + * @errp can then be used without worrying about the argument being + * NULL or &error_fatal. + * + * Using it when it's not needed is safe, but please avoid cluttering + * the source with useless code. + * + * = Converting to ERRP_GUARD() = + * + * To convert a function to use ERRP_GUARD(): + * + * 0. If the Error ** parameter is not named @errp, rename it to + * @errp. + * + * 1. Add an ERRP_GUARD() invocation, by convention right at the + * beginning of the function. This makes @errp safe to use. + * + * 2. Replace &err by errp, and err by *errp. Delete local variable + * @err. + * + * 3. Delete error_propagate(errp, *errp), replace + * error_propagate_prepend(errp, *errp, ...) by error_prepend(errp, ...) + * + * 4. Ensure @errp is valid at return: when you destroy *errp, set + * errp = NULL. + * + * Example: + * + * bool fn(..., Error **errp) + * { + * Error *err = NULL; + * + * foo(arg, &err); + * if (err) { + * handle the error... + * error_propagate(errp, err); + * return false; + * } + * ... + * } + * + * becomes + * + * bool fn(..., Error **errp) + * { + * ERRP_GUARD(); + * + * foo(arg, errp); + * if (*errp) { + * handle the error... + * return false; + * } + * ... + * } */ #ifndef ERROR_H @@ -287,6 +365,7 @@ void error_setg_win32_internal(Error **errp, * the error object. * Else, move the error object from @local_err to *@dst_errp. * On return, @local_err is invalid. + * Please use ERRP_GUARD() instead when possible. * Please don't error_propagate(&error_fatal, ...), use * error_report_err() and exit(), because that's more obvious. */ @@ -298,6 +377,7 @@ void error_propagate(Error **dst_errp, Error *local_err); * Behaves like * error_prepend(&local_err, fmt, ...); * error_propagate(dst_errp, local_err); + * Please use ERRP_GUARD() and error_prepend() instead when possible. */ void error_propagate_prepend(Error **dst_errp, Error *local_err, const char *fmt, ...); @@ -396,6 +476,46 @@ void error_set_internal(Error **errp, GCC_FMT_ATTR(6, 7); /* + * Make @errp parameter easier to use regardless of argument value + * + * This macro is for use right at the beginning of a function that + * takes an Error **errp parameter to pass errors to its caller. The + * parameter must be named @errp. + * + * It must be used when the function dereferences @errp or passes + * @errp to error_prepend(), error_vprepend(), or error_append_hint(). + * It is safe to use even when it's not needed, but please avoid + * cluttering the source with useless code. + * + * If @errp is NULL or &error_fatal, rewrite it to point to a local + * Error variable, which will be automatically propagated to the + * original @errp on function exit. + * + * Note: &error_abort is not rewritten, because that would move the + * abort from the place where the error is created to the place where + * it's propagated. + */ +#define ERRP_GUARD() \ + g_auto(ErrorPropagator) _auto_errp_prop = {.errp = errp}; \ + do { \ + if (!errp || errp == &error_fatal) { \ + errp = &_auto_errp_prop.local_err; \ + } \ + } while (0) + +typedef struct ErrorPropagator { + Error *local_err; + Error **errp; +} ErrorPropagator; + +static inline void error_propagator_cleanup(ErrorPropagator *prop) +{ + error_propagate(prop->errp, prop->local_err); +} + +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(ErrorPropagator, error_propagator_cleanup); + +/* * Special error destination to abort on error. * See error_setg() and error_propagate() for details. */ |